1. Field of the Invention
The present invention relates generally to semiconductor processing. More specifically, the present invention relates to a structure and method for forming an unlanded via.
2. Description of Related Art
During manufacturing of a semiconductor integrated circuit, electrically conductive materials patterned in electrical circuitry are layered over a base transistor structure that is disposed on a semiconductor substrate. The electrically conductive materials, such as copper, are in different and noncontiguous planes. Vias or pathways connect the various layers of electrically conductive materials. An insulator or a dielectric material is placed between the separate planes of conductive material around the vias and also within the trenches and the circuit pattern of a layer of conductive material. Vias are usually formed as landed vias, which rest entirely on a conductive layer, and not as unlanded vias, which rest partially on a conductive layer and partially on an insulator.
The demand for closer packing of devices means increased packing density of vias. Unlanded vias are preferable to landed vias in order to achieve a higher packing density. In addition, because better device performance can be achieved with lower capacitance between conductive lines within an insulating layer, the industry is moving toward using insulators with a lower dielectric constant (K), such as an organic polymer (Kxcx9c2.5) instead of silicon dioxide (Kxcx9c4.0). Thus, the next generation of chip sets requires unlanded vias and low dielectric constant materials as intraline dielectrics.
FIG. 1 is a cross-section illustrating an interconnect structure in which two landed vias 111 and 113 are formed according to a prior art method. Metal lines 100 with oxide caps 103 are patterned and formed. A polymer 105 is then deposited and filled around the metal lines 100. Polymer 105 is polished back using chemical-mechanical polish until a top surface of polymer 105 is flush with a top surface of oxide cap 103. A thin passivation layer 107 is then deposited. Next, an interlayer dielectric 109, such as silicon dioxide, is deposited. Standard lithography is used to form landed vias 111 and 113. Landed vias 111 and 113 are created by etching through interlayer dielectric 109, through passivation layer 107, and through oxide cap 103 until landed vias 111 and 113 contact metal lines 100.
The same process is then used to create unlanded vias 200 and 202 in FIG. 2. Unfortunately, the prior art method shown in FIG. 1 cannot be used to create unlanded vias 200 and 202. During the etch of unlanded vias 200 and 202, polymer 105 has been punched through, which leads to device reliability problems. In addition, unlanded vias 200 and 202 cannot be filled with a metal, such as tungsten, because tungsten cannot nucleate on a polymer 105. Thus, a void is created and device performance is degraded. Thus, the prior art method cannot be used to create unlanded vias 200 and 202 as shown in FIG. 2 in the presence of a polymer. Therefore, the prior art method fails to allow the fabrication of unlanded vias over a metal and a polymer that is used as an intraline dielectric.
The present invention relates to a method for forming an interconnect structure. The method comprises the following steps. A conductive structure having an insulator cap is formed. A low dielectric constant material is formed over the conductive structure. The low dielectric constant material is planarized. The low dielectric constant material is then selectively removed to create a recess. A stop layer is formed over the recess of the low dielectric constant material and over the conductive structure. The stop layer is then planarized.